1. Field of the Invention
The present invention relates generally to a combined cycle gas turbine system and, more particularly, to a combined cycle gas turbine system in which the temperature and flow rate of cooling steam are efficiently controlled and the heating of fuel and the cooling of gas turbine blade cooling air are carried out by steam generated at a waste heat recovery boiler.
2. Description of the Prior Art
FIG. 10 is a diagram of a steam cooled type combined cycle gas turbine system in the prior art.
In FIG. 10, the prior art steam cooled gas turbine system is constructed by a gas turbine 8, a waste heat recovery boiler 9 and a steam turbine 29. In the gas turbine 8, suction air is taken into a compressor 2 to be compressed to a predetermined pressure, and while the compressed air is partially used for cooling a gas turbine blade, the greater part thereof is led into a combustor 3 to be mixed with fuel 7 for generation of a high temperature gas. The high temperature gas enters a turbine 6 to expand for work, and the turbine output, after deduction of the compressor output, is converted into electric power at a generator 1. On the other hand, outlet steam of a high pressure turbine 21 flowing through piping 101 is partially taken to be supplied into the turbine 6 for cooling the gas turbine blade via the cooling steam supply piping 101. This steam is heated by cooling a steam cooled blade 51 and is recovered into an inlet of an intermediate pressure turbine 22 via cooling steam recovery piping 102. Thus, for cooling the gas turbine blade, the air bled from the compressor 2 and a portion of the outlet steam of the high pressure turbine 21 are used.
While outlet air of the compressor 2 is partially used for blade cooling in the turbine 6, this air, being of a high temperature, is cooled to a predetermined temperature at a blade cooling air cooler 4a using a cooling fan 5 and is then used for the turbine blade cooling. Thus, the air so led from the compressor 2 is first cooled at the blade cooling air cooler 4a using the cooling fan 5 to be then supplied into the turbine 6.
In the waste heat recovery boiler 9, outlet steam of a low pressure turbine 23 is converted into water from steam at a condenser 25. Then, the water is pressurized at a feed water pump 26 and heated at a feed water heater 10 to become saturated water. This saturated water is separated into three systems of water. The first one becomes saturated steam at a low pressure evaporator 11, becomes superheated steam at a low pressure superheater 15 and is then supplied to an inlet of the low pressure turbine 23. The second one is pressurized to a predetermined pressure at an intermediate pressure pump 28, becomes saturated water at an intermediate pressure economizer 12, becomes saturated steam at an intermediate pressure evaporator 14, becomes superheated steam at an intermediate pressure superheater 16 and is then supplied to an inlet of a reheater 20. And the third one is pressurized to a predetermined pressure at a high pressure pump 27, becomes saturated water at a first high pressure economizer 13 and a second high pressure economizer 17, becomes saturated steam at a high pressure evaporator 18, becomes superheated steam at a high pressure superheater 19 and is then led into the high pressure turbine 21. The mentioned superheated steam enters the high pressure turbine 21, the intermediate pressure turbine 22 and the low pressure turbine 23, respectively, to expand for generating an output, and this output is converted into electric power at a generator 24.
With respect to the abovementioned cooling by steam, it is impossible to use the steam in a quantity in excess of that of the steam obtainable at the outlet of the high pressure turbine 21. Hence, in order to secure a spare quantity of the available steam, it is preferable to reduce the flow rate of the cooling steam to the extent possible. Also, if the cooling steam is made less in quantity, it becomes possible to control the temperature of the steam, after being used for cooling, with less variation in the quantity of the cooling steam. Especially, if the temperature of the cooling steam heated by cooling is maintained to a predetermined level, it will not only enhance the reliability and life of the cooled blade, rotor, pipings, etc. of the gas turbine, but it will also ensure an operation that does not damage the enhanced combined efficiency. In order to reduce the quantity of the cooling steam, it is necessary to reduce the temperature of the cooling steam.
Thus, while the temperature of the cooling steam is necessary to be maintained lower for enhancing the reliability of the cooled blade or the like, in the system shown in FIG. 10, the cooling steam supply temperature is decided by the outlet conditions of the high pressure turbine 21, and it is difficult to further reduce the cooling steam temperature in this system.
Also, the air bled from the compressor for cooling the gas turbine blade is once cooled at the blade cooling air cooler 4a using the cooling fan 5 to be supplied into the turbine 6, as mentioned above, and the heat obtained by such cooling is discharged outside without use. This causes a reduction in the thermal efficiency (gas turbine efficiency and combined efficiency) of the gas turbine and of a combined cycle system using this gas turbine. Moreover, the fuel 7 is supplied into the combustor 3 without being heated (preheated).